Thixotropic fluorinated curable compositions and articles sealed therewith

ABSTRACT

In fluorinated curable compositions comprising alkenyl group-bearing perfluoropolyethers as the base polymer to ensure chemical resistance and solvent resistance together with an organohydrogensiloxane and a platinum group metal catalyst, a hydrophobized silica fine powder is used in combination therewith to render the compositions thixotropic.

[0001] This invention relates to fluorinated curable compositions of theaddition curing type having high thixotropy, curing into gel productshaving improved chemical resistance and solvent resistance, and thussuitable for use as potting, sealing and coating materials for electricand electronic parts, and a variety of articles sealed with the curedcompositions.

BACKGROUND OF THE INVENTION

[0002] Cured gel products of silicone rubber have good electrically andthermally insulating properties, stable electric properties andflexibility. They are useful as potting and sealing materials forelectric and electronic parts, and coating materials for protectingcontrol circuit devices such as power transistors, ICs and capacitorsfrom external, thermal and mechanical damages.

[0003] Typical silicone compositions capable of forming such cured gelproducts are organopolysiloxane compositions of the addition curingtype. The organopolysiloxane compositions of the addition curing typeare known, for example, from JP-A 56-143241, 62-3959, 63-35655 and63-33475 as comprising an organopolysiloxane having vinyl groupsattached to silicon atoms and an organohydrogenpolysiloxane havinghydrogen atoms attached to silicon atoms, wherein crosslinking reactiontakes place in the presence of a platinum group catalyst to form asilicone gel.

[0004] Also known in the art are fluorosilicone gel compositionscomprising an organopolysiloxane having trifluoropropyl groups (see JP-A7-324165).

[0005] However, these organopolysiloxane compositions of the additioncuring type result in silicone gels which are prone to degradation orswelling upon exposure to such chemicals as strong bases and strongacids or such solvents as toluene, alcohols and gasoline, often failingto maintain their performance.

[0006] As one solution to this problem, JP-A 11-116685 discloses afluorochemical gel composition comprising a polyfluoro compound havingtwo alkenyl groups and a divalent perfluoropolyether group in amolecule, an organohydrogen-polysiloxane having hydrogen atoms attachedto silicon atoms, and a platinum group catalyst as well as afluorochemical gel product resulting from curing thereof.

[0007] When gel products are used for protection purpose, gelcompositions having a gel-forming ability must be applied onto surfacesof electronic parts by spot potting. However, prior art gel compositionsare not amenable to spot potting on account of fast flow, failing toform an effective protective layer on electronic parts. Therefore, thereis a need for a gel composition which is reasonably flowable andresistant to chemicals and solvents.

[0008] The fluidity of gel compositions can be reduced by increasingtheir viscosity. However, the compositions lower their viscosity uponheating for curing, yet failing to form an effective protective layer onelectronic parts.

SUMMARY OF THE INVENTION

[0009] Therefore, an object of the invention is to provide a fluorinatedcurable composition which can form a flexible cured gel product and isso thixotropic that it is amenable to potting due to a low apparentviscosity when shear stresses are applied during potting operation bymeans of a dispenser or the like, but it does not substantially flow dueto a high apparent viscosity when no shear stresses are applied. Anotherobject of the invention is to provide an article, such as an electric orelectronic part, sealed with the cured product of the composition.

[0010] The invention provides a thixotropic fluorinated curablecomposition comprising

[0011] (A) a linear polyfluoro compound of the general formula (1):

CH₂═CH—(X)_(a)—Rf¹—(X′)_(a)—CH═CH₂  (I)

[0012] wherein X is —CH₂—, —CH₂O—, —CH₂OCH₂— or —Y—NR¹—CO—, Y is

[0013] —CH₂— or a group of the following structural formula (Z):

[0014] X′ is —CH₂—, —OCH₂—, —CH₂OCH₂— or —CO—NR¹—Y′—, Y′ is —CH₂— or agroup of the following structural formula (Z′):

[0015] R¹ is hydrogen, methyl, phenyl or allyl, Rf¹ is a divalentperfluoropolyether group, and “a” is independently 0 or 1,

[0016] (B) an organohydrogensiloxane having at least two hydrogen atomseach attached to a silicon atom in a molecule,

[0017] (C) an effective amount of a platinum group metal catalyst,

[0018] (D) finely divided silica hydrophobized with a polysiloxane andhaving a specific surface area of at least 50 m²/g, and optionally,

[0019] (E) a second linear polyfluoro compound of the following generalformula (2):

Rf²—(X)_(a)—CH═CH₂  (2)

[0020] wherein X and “a” are as defined above, and Rf² is a monovalentperfluoropolyether group.

[0021] Also contemplated herein are an electric or electronic part,pressure sensor, rotation sensor and air flow meter which is sealed withthe thixotropic fluorinated curable composition in the cured state.

[0022] It has been found that the above and other objects areeffectively achieved by combining components (A) through (D) orcomponents (A) through (E), and especially by compounding silica (D) inan amount of 0.5 to 8 parts by weight per 100 parts by weight ofcomponent (A) or components (A) and (E) combined, because the resultingcomposition is satisfactorily thixotropic and capable of forming aflexible cured gel product.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] In the fluorinated curable composition of the invention,component (A) is a linear polyfluoro compound of the following generalformula (1).

CH₂═CH—(X)_(a)—Rf¹—(X′)_(a)—CH═CH₂  (1)

[0024] Herein X is —CH₂—, —CH₂O—, —CH₂OCH₂— or —Y—NR¹—CO— wherein Y is—CH₂— or a group of the following structural formula (Z):

[0025] and R¹ is hydrogen, methyl, phenyl or allyl.

[0026] X′ is —CH₂—, —OCH₂—, —CH₂OCH₂— or —CO—NR¹—Y′— wherein Y′ is —CH₂—or a group of the following structural formula (Z′):

[0027] and R¹ is as defined above.

[0028] Rf¹ is a divalent perfluoropolyether group, and “a” isindependently equal to 0 or 1.

[0029] In formula (1), Rf¹ is a divalent perfluoropolyether group, andpreferably selected from groups of the following formulae:

[0030] wherein p and q each are an integer of at least 1, the sum of p+qis from 2 to 200, and r is an integer of 2 to 6,

[0031] wherein r is an integer of 2 to 6, s is an integer inclusive of0, t is an integer inclusive of 0, and the sum of s+t is from 0 to 200,and

[0032] wherein u is an integer of 1 to 200 and v is an integer of 1 to50.

[0033] Illustrative examples of Rf¹ are given below.

[0034] In formula (1), X is independently —CH₂—, —CH₂O—, —CH₂OCH₂— or—Y—NR¹—CO—, and X′ is —CH₂—, —OCH₂—, —CH₂OCH₂— or —CO—NR¹—Y′—. Herein Yis —CH₂— or a group of the formula (Z), and Y′ is —CH₂— or a group ofthe formula (Z′).

[0035] R′ is hydrogen, methyl, phenyl or allyl.

[0036] The alkenyl groups in formula (1) are preferably vinyl and allylgroups having a —CH═CH₂ structure at the end. The alkenyl groups may bebonded to opposite ends of the backbone directly or via divalent linkagegroups represented by X or X′.

[0037] In formula (1), letter “a” is independently equal to 0 or 1.

[0038] It is preferred that the inventive composition have anappropriate flow so that the composition is useful in casting, potting,coating, impregnation and adhesion, and that its cured product haveappropriate physical properties. From this standpoint, component (A)should preferably have a viscosity at 23° C. in the range of 5 to100,000 mPa·s, especially 500 to 50,000 mPa·s. Depending on a particularapplication, component (A) having a most appropriate viscosity withinthe range is desirably used.

[0039] Component (B) is an organohydrogensiloxane having at least twohydrogen atoms each attached to a silicon atom in a molecule.

[0040] The organohydrogensiloxane (B) serves as a crosslinker and chainextender for component (A) and optional component (E), and should haveat least two hydrosilyl groups (i.e., Si—H groups) in a molecule. Forcompatibility and dispersion with components (A) and (E) and cureduniformity, component (B) should preferably have at least onefluorinated group.

[0041] Suitable fluorinated groups include monovalent perfluorooxyalkylgroups, monovalent perfluoroalkyl groups, divalent perfluorooxyalkylenegroups and divalent perfluoroalkylene groups. These perfluorooxyalkyl,perfluoroalkyl, perfluorooxyalkylene and perfluoroalkylene groups arepreferably those of the following general formulae.

[0042] monovalent perfluoroalkyl groups:

C_(m)F_(2m+1)—

[0043] m is an integer of 1 to 20, preferably 2 to 10.

[0044] divalent perfluoroalkylene groups:

—C_(m)F_(2m)—

[0045] m is an integer of 1 to 20, preferably 2 to 10.

[0046] monovalent perfluorooxyalkyl groups:

[0047] n is an integer of 1 to 5.

[0048] divalent perfluorooxyalkylene groups:

[0049] An average of m+n is an integer of 2 to 100.

[0050] The following compounds are exemplary of the fluorinatedorganohydrogensiloxane. These compounds may be used alone or inadmixture of two or more. In the formulae, Me is methyl and Ph isphenyl.

[0051] Each of n and m is an integer of at least 1, and n+m is from 2 to50.

[0052] Component (B) is included in such amounts that 0.2 to 2 moles,more preferably 0.5 to 1.3 moles of hydrosilyl groups (i.e., Si—Hgroups) are available from component (B) per mole of aliphaticunsaturated groups (e.g., vinyl and allyl groups) in the entirecomposition. Amounts of component (B) to provide less than 0.2 mole ofSi—H groups may fail to drive crosslinking to a sufficient degree toform a cured gel product. Amounts of component (B) to provide more than2 moles of Si—H groups may invite a risk of foaming upon curing.

[0053] Component (C) is a platinum group metal catalyst for promotingaddition reaction between aliphatic unsaturated groups in components (A)and (E) and hydrosilyl groups in component (B). These catalysts aregenerally noble metal compounds which are expensive, and therefore,platinum compounds which are relatively easily available are oftenemployed.

[0054] The platinum compounds include, for example, chloroplatinic acid,complexes of chloroplatinic acid with olefins such as ethylene,complexes of chloroplatinic acid with alcohols and vinylsiloxanes, andplatinum on silica, alumina or carbon, though not limited thereto. Knownexamples of the platinum group compounds other than the platinumcompound are rhodium, ruthenium, iridium and palladium compounds, forexample, RhCl(PPh₃)₃, RhCl(CO)(PPh₃)₂, Ru₃(CO),₂, IrCl(CO)(PPh₃)₂, andPd(PPh₃)₄.

[0055] The catalyst is used in a catalytic amount. Usually, thepreferred amount of the catalyst is about 0.1 to 100 parts by weight ofplatinum group metal per million parts by weight of components (A), (B)and (E) combined.

[0056] Component (D) is finely divided silica. This filler componentplays the important role of imparting thixotropy to the compositionprior to curing. Since the filler component will not be extracted withsolvents or contracted after curing, it does not adversely affect theproperties of electronic parts when the composition is used as aprotective material for the parts.

[0057] In order that finely divided silica interact with components (A)and (E) to exert thixotropic effects, the finely divided silica shouldhave a specific surface area of at least 50 m²/g, preferably 50 to 400m²/g and be surface hydrophobized. With the lack of either of theserequirements, the desired thixotropy is not imparted. The hydrophobictreatment is to treat with polysiloxanes, especially having methylgroups. Suitable surface treating agents include cyclic and linearpolydimethylsiloxanes, which may be used alone or in admixture of any.After the silica filler is surface treated to be hydrophobic, the amountof carbon on the treated silica surface is preferably 0.3 to 8% byweight, especially 2 to 6% by weight, based on the entire hydrophobizedsilica. The surface treated silica is desirably of high purity becausethe purity of the composition, which is crucial in the application toelectric and electronic parts, largely depends on the purity of thesilica.

[0058] Commercial products may be used as the finely divided silica ofhigh purity, for example, Cabosil TS-720 (Cabot Corp.).

[0059] Finely divided silica is desirably included in amounts of 0.5 to8 parts by weight, more desirably 2 to 6 parts by weight, per 100 partsby weight of components (A) and (E) combined. Less than 0.5 part ofsilica may fail to render the composition fully thixotropic with therisk of the uncured composition flowing away upon spot potting. Morethan 8 parts of silica may increase a viscosity and adversely affectworkability. Most preferably, finely divided silica is included in suchamounts for the composition to have a thixotropy index of 1.5 to 3.5,especially 2.0 to 3.2. It is noted that the thixotropy index is a ratioof the apparent viscosity at a low rotation speed (e.g., 4 to 12 rpm) tothe apparent viscosity at a high rotation speed (e.g., 20 to 60 rpm),provided that a ratio of the high rotation speed to the low rotationspeed is at least 5, preferably in the range from 5 to 10.

[0060] Component (E) is a second linear polyfluoro compound of thefollowing general formula (2):

Rf²—(X)_(a)—CH═CH₂  (2)

[0061] wherein X and “a” are as defined above, and Rf² is a monovalentperfluoropolyether group.

[0062] Like component (A), the alkenyl groups in formula (2) arepreferably vinyl and allyl groups having a —CH═CH₂ structure at the end.The alkenyl groups may be bonded to the backbone directly or viadivalent linkage groups represented by X.

[0063] In formula (2), Rf² is a monovalent perfluoropolyether group, andpreferably selected from groups of the following formulae:

[0064] wherein p is an integer of at least 1, and

[0065] wherein q is an integer of at least 1.

[0066] Illustrative examples of Rf² are given below.

[0067] In formula (2), X and “a” are as defined in formula (1). X and“a” in formula (1) may be identical with or different from X and “a” informula (2).

[0068] Component (E) is preferably included in amounts of 0 to 200 partsby weight, more preferably 50 to 180 parts, and most preferably 80 to150 parts by weight, per 100 parts by weight of component (A). Inclusionof component (E) facilitates adjustment of physical properties of thecured composition, inter alia, penetration and has the additionaladvantage that the cured composition becomes thermally stable. Toenhance such effects, component (E), when used, is preferably includedin amounts of at least 80 parts by weight.

[0069] In addition to components (A) to (E), the inventive compositionmay contain various additives known to those skilled in the art. Suchoptional additives include regulators for hydrosilylation catalysts, forexample, acetylene alcohols such as 1-ethynyl-1-hydroxycyclohexane,3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol,3-methyl-1-penten-3-ol, and phenylbutynol, 3-methyl-3-penten-1-yne,3,5-dimethyl-3-hexen-1-yne, etc., polymethylvinylsiloxane cycliccompounds, and organic phosphorus compounds. These regulators areeffective for maintaining curing reactivity and storage stabilityappropriate. Other optional components include inorganic fillers, forexample, iron oxide, zinc oxide, titanium oxide, calcium carbonate,magnesium carbonate, zinc carbonate, and carbon black. By adding suchinorganic fillers to the inventive composition, the hardness andmechanical strength of cured gel can be adjusted. There may be addedhollow inorganic fillers and rubbery spherical fillers as well. Also, byadding a well compatible perfluoropolyether oil free of reactivefunctional groups, the viscosity of the composition or the hardness of acured product can be adjusted. The amounts of these optional componentsadded are arbitrary insofar as they do not compromise the properties ofthe curable composition and the physical properties of cured gel.

[0070] The fluorinated curable composition comprising the essential andoptional components mentioned above is cured into a gel product (orcured gel) having improved solvent resistance and chemical resistance.

[0071] By the term “cured gel” as used herein it is meant that the curedproduct has a three-dimensional structure in part and undergoesdeformation and flow under stress. As an approximate measure, the curedgel has a hardness of up to “0” as measured by a JIS rubber hardnessmeter or a penetration of 1 to 200 as measured according to ASTM D-1403(1/4 cone).

[0072] The cured gel is generally obtained by well-known methods, forexample, by pouring the inventive composition of the addition curingtype into a suitable mold and causing the composition to cure therein.Alternatively, the inventive composition is coated onto a suitablesubstrate and cured thereon. Curing is effected simply by heating at atemperature of about 60 to 150° C. for about 30 to 180 minutes.

[0073] Electrical and electronic parts can be sealed with the curedcomposition and include various sensors, for example, pressure sensors(e.g., gas pressure sensors and hydraulic pressure sensors), temperaturesensors, rotation sensors, and timing sensors as well as air flow metersand various control units.

[0074] There have been described fluorinated curable compositions inwhich components (A) and (E), alkenyl group-bearing perfluoropolyethersare used as the base polymer of a gel composition to ensure chemicalresistance and solvent resistance and component (D), hydrophobizedsilica fine powder is used in combination therewith to render the gelcomposition thixotropic.

EXAMPLE

[0075] Examples of the invention are given below by way of illustrationand not by way of limitation. Parts are by weight.

Example 1

[0076] To 40 parts of a polymer (viscosity 2,100 cSt) of the followingformula (3) and 60 parts of a polymer (viscosity 650 cSt) of thefollowing formula (4) was added 3.0 parts of Cabosil TS-720 (Cabot),which is polysiloxane-treated silica having a specific surface area of100 m²/g and a carbon content on hydrophobic surface of 4.5% by weightcalculated as carbon, as the finely divided silica. They were mixed in aplanetary mixer until uniform, and further heat treated at 150° C. for 2hours. The mixture was cooled down to room temperature, after which wereadded 0.15 part of a 50% toluene solution of ethynylcyclohexanol, 0.015part of an ethanol solution of a chloroplatinic acid-vinylsiloxanecomplex (platinum concentration 3.0% by weight), and 14 parts of acompound of the following formula (5). Mixing these components yielded afluorinated composition.

[0077] Using a rotational viscometer, the viscosity of the compositionwas measured at a rotational speed of 6 rpm and 30 rpm, and a thixotropyindex was computed from these viscosities. The composition was cured byheating at 150° C. for one hour, obtaining a cured gel whose penetrationwas measured according to ASTM D-1403 (1/4 cone). The results are shownin Table 1.

Example 2

[0078] A composition was prepared as in Example 1 except that the amountof finely divided silica was changed to 4.0 parts. The viscosity andthixotropy index of the composition and the penetration of the gel weremeasured as in Example 1, with the results also shown in Table 1.

Comparative Example 1

[0079] A composition was prepared as in Example 1 except that the amountof finely divided silica was changed to 0.3 part. The viscosity andthixotropy index of the composition and the penetration of the gel weremeasured as in Example 1, with the results also shown in Table 1.

Comparative Example 2

[0080] A composition was prepared as in Example 1 except that the finelydivided silica used in Example 1 was changed to 3.0 parts of AerosilR974 (Nippon Aerosil Co., Ltd.), which is dimethyldichlorosilane-treatedsilica having a specific surface area of 170 m²/g and a carbon contenton hydrophobic surface of 1% by weight calculated as carbon. Theviscosity and thixotropy index of the composition and the penetration ofthe gel were measured as in Example 1, with the results also shown inTable 1. TABLE 1 Comparative Example Example 1 2 1 2 Viscosity @6 rpm(Pa · s) 22.7 40.5 10.3 12.7 Viscosity @30 rpm (Pa · s) 9.0 13.1 7.5 9.1Thixotropy index 2.5 3.1 1.4 1.4 Penetration 58 44 61 54

[0081] A quantity (3 g) of each composition of Examples 1, 2 andComparative Examples 1, 2 was spilled on a glass plate. The compositionsof Examples 1 and 2 retained their shape whereas the compositions ofComparative Examples 1 and 2 spread out without retaining shape.

[0082] Japanese Patent Application No. 2001-389406 is incorporatedherein by reference.

[0083] Reasonable modifications and variations are possible from theforegoing disclosure without departing from either the spirit or scopeof the present invention as defined by the claims.

1. A thixotropic fluorinated curable composition comprising (A) a linearpolyfluoro compound of the general formula (1):CH₂═CH—(X)_(a)—Rf¹—(X′)_(a)—CH═CH₂  (1)wherein X is —CH₂—, —CH₂O—,—CH₂OCH₂— or —Y—NR¹—CO—, Y is —CH₂— or a group of the followingstructural formula (Z):

X′ is —CH₂—, —OCH₂—, —CH₂OCH₂— or —CO—NR¹—Y′—, Y′ is —CH₂— or a group ofthe following structural formula (Z′):

R¹ is hydrogen, methyl, phenyl or allyl, Rf¹ is a divalentperfluoropolyether group, and “a” is independently 0 or 1, (B) anorganohydrogensiloxane having at least two hydrogen atoms each attachedto a silicon atom in a molecule, (C) an effective amount of a platinumgroup metal catalyst, and (D) finely divided silica hydrophobized with apolysiloxane and having a specific surface area of at least 50 m²/g. 2.The curable composition of claim 1, wherein Rf¹ in formula (1) is

wherein p and q each are an integer of at least 1, the sum of p+q is 2to 200, and r is an integer of 2 to 6,

wherein r is an integer of 2 to 6, s is an integer of at least 0, t isan integer of at least 0, and the sum of s+t is 0 to 200, or

wherein u is an integer of 1 to 200 and v is an integer of 1 to
 50. 3.The curable composition of claim 1, further comprising (E) a secondlinear polyfluoro compound of the following general formula (2):Rf²—(X)_(a)—CH═CH₂  (2)wherein X and “a” are as defined above, and Rf²is a monovalent perfluoropolyether group.
 4. The curable composition ofclaim 3, wherein Rf² in formula (2) is

wherein p is an integer of at least 1, or

wherein q is an integer of at least
 1. 5. The curable composition ofclaim 1, wherein the finely divided silica (D) is included in an amountof 0.5 to 8 parts by weight per 100 parts by weight of component (A) orcomponents (A) and (E) combined.
 6. The curable composition of claim 1,having a thixotropy index of 1.5 to 3.5.
 7. The curable composition ofclaim 1, which cures into a product having a penetration of 1 to 200 asmeasured according to ASTM D-1403.
 8. An electric or electronic partsealed with the thixotropic fluorinated curable composition of claim 1in the cured state.
 9. A pressure sensor sealed with the thixotropicfluorinated curable composition of claim 1 in the cured state.
 10. Arotation sensor sealed with the thixotropic fluorinated curablecomposition of claim 1 in the cured state.
 11. An air flow meter sealedwith the thixotropic fluorinated curable composition of claim 1 in thecured state.